Tractor trailer anti-trailer swing system and method
BACKGROUND OF THE INVENTION 
     1. Related Applications 
     This application is related to U.S. Ser. No. 454,109 titled Trailer Brake Anti-Swing System and Method, U.S. Ser. No. 454,096 titled Articulation Angle Sensor, U.S. Ser. No. 454,110 titled Trailer Anti-Swing System and Method, U.S. Ser. No. 454,095 titled Anti-Trailer Swing Control and U.S. Ser. No. 454,574 titled Tractor Trailer Articulation Control System and Method, all assigned to the assignee of this application and filed the same day (Dec. 20, 1989) as this application. 
     2. Field of the Invention 
     The present invention relates to a control system/method for controlling the braking force applied to the brakes of a towed subvehicle in an articulated vehicle system, such as the semitrailer subvehicle in a tractor-semitrailer system, to prevent, arrest or to minimize and quickly recover from, the condition known as trailer brake induced trailer swing. 
     DESCRIPTION OF THE PRIOR ART 
     Brake control systems for all types of vehicles, including heavy duty tractor-semitrailer trucks, to improve the stopping and vehicle stability thereof are, of course, well known in the prior art. 
     Brake systems of the anti-lock type, for all types of vehicles, are well known in the prior art. Briefly, these systems operate to maintain vehicle stability (i.e. acceptable transverse coefficient of friction of braked wheels) by maintaining the longitudinal slip of the braked wheels within predetermined limits. This usually requires modulating the braking forces on an individual wheel and/or individual axle basis to maintain at least some wheel rotation. 
     Examples of prior art anti-lock brake systems (&#34;ABSs&#34;) may be seen by reference to U.S. Pat. Nos. 3,767,270; 3,768,872; 3,854,556; 3,893,696; 3,929,383; 3,929,382; 3,966,267; 4,392,202 and 4,591,213, the disclosures of all of which are hereby incorporated by reference. 
     Brakes systems which control braking to achieve a driver demand, sense driver demand in a &#34;brake-by-wire&#34; manner, sense coefficient of friction and modify brake forces accordingly, sense load on a wheel and modify braking effort accordingly, sense wheel slip and/or use electronic signals to achieve trailer brake response are also disclosed in the prior art as may be seen by reference to U.S. Pat. Nos. 4,140,352; 4,327,414; 4,494,199; 4,512,615; 4,545,240; 4,591,213; 4,606,586; 4,616,881; 4,648,663 and 4,768,840, the disclosures of which are hereby incorporated by reference. 
     Brake systems for heavy duty articulated vehicles such as tractor-semitrailer trucks are difficult to design as the loading and maintenance will vary in a truck, such as the loading on the tractor of a tractor-trailer which may comprise a tractor only, a tractor with an empty or lightly loaded trailer or a tractor with a heavily loaded trailer. 
     Further, tractor-semitrailers are, by their nature, capable of exhibiting certain unstable dynamic behaviors known as jackknife and trailer swing, each of which has its own characteristic cause, effect and appropriate sequence of corrective action. Jackknife is sometimes called &#34;tractor brake caused jackknife&#34; while trailer swing is sometimes called &#34;trailer brake caused jackknife&#34;. 
     The dynamics of undesirable trailer articulation events, such as a jackknife event, are discussed in SAE Paper No. 710045, the disclosure of which is hereby incorporated by reference. 
     Various systems to prevent or minimize undesirable trailer articulation have been proposed. These include mechanical devices such as chains or variable pivot resistance devices and also wheel speed sensors and anti-lock controls on the trailer per se. Examples of these prior art devices may be seen by reference to U.S. Pat. Nos. 3,618,983; 3,810,521; 3,894,773; 4,023,864, 4,405,145 and 4,620,717 the disclosures of which are hereby incorporated by reference. 
     The prior art devices were not satisfactory as the mechanical devices were somewhat slow to react, hindered required articulation during normal operation, required specially equipped/configured trailers and/or did not allow for a recovery from the locked-in condition. The anti-lock (ABS) type systems were not totally satisfactory as most existing trailers do not have ABS equipment, tractors are often driven with a variety of trailers and thus even if a trailer is provided with ABS equipment, it may not be compatible with the tractor ABS, providing all existing and future trailers with ABS equipment is prohibitively expensive and, under certain conditions, undesirable trailer articulation may occur in the absence of the wheel lock conditions sensed and reacted to by existing ABSs. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, many of the drawbacks of the prior art have been overcome or minimized by the provision of a trailer anti-swing control system/method which is suitable for use with semitrailers equipped with standard (i.e., non-ABS) brake controls and will control the pilot or actuation pressure supplied to the trailer brake system to halt sensed trailer swing and allow the trailer to safely recover therefrom. 
     The above is accomplished by providing, preferably entirely on the tractor, an ABS type valve located on the tractor upstream from the gladhand connection, sensors for sensing and/or providing signals allowing the calculation of tractor/trailer articulation angle (&#34;AA&#34;) and rate of change of rate of change of articulation angle (d 2  AA/dt 2 ) and a control device, preferably microprocessor based, including a timer, for receiving input signals, processing same in accordance with predetermined logic rules and issuing command output signals to control the trailer brake ABS type valve. 
     For a more responsive control, higher order time derivatives of articulation angle may be used. The &#34;ABS type valve&#34; should have the functions of lowering pressure to release the trailer brakes, holding the pressure at a low value to maintain brakes released, then reapplying the brakes in a controlled manner to a desired reapply pressure, all independent of the degree of braking effort demanded by the vehicle driver. 
     Accordingly, it is an object of the present invention to provide an improved trailer brake control system/method for the trailer type subvehicle of an articulated vehicle system, such as the trailer of a tractor-semitrailer truck system, which is operative to detect the onset of and prevent, or to minimize and allow recovery from, a trailer swing condition. Preferably, all of the control components, at least the active control components, will be mounted on the tractor subvehicle and will allow use of the control system with any standardly equipped trailer subvehicle. 
     This and other objects and advantages of the present invention will become apparent from a reading of the detailed disclosure of the preferred embodiments taken in connection with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a conventional heavy duty tractor-semitrailer truck articulated vehicle. 
     FIG. 2 is a schematic illustration of a tractor mounted brake control system including an anti-lock brake system, and also incorporating the trailer anti-swing control system of the present invention. 
     FIG. 3 is a schematic illustration of a standard trailer brake system which may be utilized with the trailer anti-swing brake control system of the present invention. 
     FIG. 4 is an enlarged, fragmentary, schematic illustration of the king pin/fifth wheel pivotal connection between a tractor and a semitrailer illustrating the angular relationship defined by the articulation angle. 
     FIG. 5 is a graph illustrating the trailer brake applied pressure versus time curve(s) for the trailer anti-swing control system/method of the present invention. 
     FIG. 6 is a schematic illustration of the occurrence of an uncontrolled trailer swing event. 
     FIG. 7 is an illustration of a floating rotor potentiometer type of articulation angle sensor for the control system/method of the present invention. 
     FIG. 8 is an illustration of an ultrasonic distance sensor type articulation angle sensor for the control system/method of the present invention. 
     FIG. 9 is a schematic illustration of a magnetic field direction sensor type of articulation angle sensor for the control system/method of the present invention. 
     FIG. 10 is a schematic illustration of a glad hand transmitter type articulation angle sensor for the control system/method of the present invention. 
     FIGS. 11 and 11A are a schematic illustration, in the form of a flow chart, of the trailer anti-swing control system/method of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The trailer anti-swing control system/method of the present invention is applicable to articulated multiple vehicle systems such as the tractor-semitrailer system 10 illustrated in FIG. 1. Briefly, as is well known to the prior art, the tractor-semitrailer system 10 comprises a tractor 12 and a semitrailer 14 attached thereto by a connecting means 16 which comprises the well known fifth wheel 34 fixed to the tractor for selective engagement with a king pin fixed to the trailer. The tractor typically comprises a pair or tandem set of rear drive axles 36 and 38 and a front steer axle 40. The trailer 14 typically comprises a tandem pair of non-steerable, non-driven trailer axles 42 and 44. Typically, but not necessarily, the front axle 40 will have non-driven steerable wheels. The tractor and/or the trailer may be equipped with single axles or three or more axles. 
     The ability to utilize fifth wheel/king pin connections to relatively quickly and easily couple or connect standardly equipped tractors to standardly equipped semitrailers in an articulated or pivoted manner provides the well appreciated advantages of commercial and operational flexibility as well as increasing the maneuverability of the vehicles. However, under certain conditions, often associated with braking while negotiating a curve or a turn, one or both of the tractor-semitrailer subvehicles may lose transverse stability resulting in a condition of uncontrolled excessive articulation, i.e. jackknife or trailer swing. Tractor-semitrailer uncontrolled excessive articulation is generally considered to consist of two distinct types of events, namely &#34;jackknife&#34; and &#34;trailer swing&#34;, both of which may lead to catastrophic results. 
     Jackknifing, which is considered to be the most severe and least correctable type of uncontrolled excessive trailer articulation event often results from the tractor subvehicle, usually the tractor subvehicle drivewheels, losing transverse stability, usually while cornering, which lack of stability is quickly compounded by the often relatively many times greater inertia imposed by the trailer. The other type of uncontrolled excessive trailer articulation event, trailer swing, is generally the result of the trailer wheels, 42 and 44, losing transverse stability, usually while cornering, resulting in the trailer swinging radially outwardly relative to the center of the curve being negotiated which may result in the trailer to swinging into adjacent traffic lanes with the obvious potential for catastrophic results therefrom. 
     Trailer swing events often occur less rapidly than the jackknife events, and upon sensing conditions indicative of the advent of a trailer swing condition corrective action may be taken of the trailer brakes to minimize the extent of trailer swing and to cause the trailer to resume its tracking condition. 
     As is very well known, decreasing the slip of a tire will dramatically increase the transverse coefficient of friction thereof. Accordingly, trailer swing induced by locked or almost locked trailer brakes, if sensed at the onset or initiation thereof, may be arrested or reduced by increasing the transverse stability of the trailer wheels by releasing the brakes thereof and allowing the wheels to roll up to or towards vehicle speed. The schematic illustration of an extreme trailer swing event may be seen by reference to FIG. 6 wherein the trailer at the right-hand portion of the illustrated curve has swung totally out of its lane. The purpose of the control system/method of the present invention is to prevent the occurrence of, or to minimize the extent of, trailer brake induced trailer swing events. 
     The trailer anti-swing control system/method of the present invention is preferably utilized with a tractor 12 fully equipped with an ABS system and a trailer 14 equipped with a standard, i.e. non-ABS, brake system. However, the control of the present invention would also provide benefits if utilized with a vehicle having both tractor and trailer ABS as, under certain conditions, undesirable trailer articulation may occur even though the vehicle wheels have not locked up to the extent that is necessary to cause corrective action by currently existing ABS systems. Additionally, although not preferred, the trailer anti-swing control system/method of the present invention will provide a degree of improved vehicle stability when utilized on a tractor trailer system wherein neither the tractor nor the trailer was provided with an ABS logic control. 
     As stated above, the purpose of the present invention is to prevent or minimize trailer brake induced trailer swing events while the occurrence of jackknife events is minimized by the provision of the tractor ABS control logic and devices. 
     A braking system for the tractor 12 of an articulated tractor-semitrailer system 10 which utilizes the trailer anti-swing control system/method of the present invention may be seen by reference to FIG. 2. It is noted that while the braking system for tractor 12 illustrated in FIG. 2 is of the &#34;brake by wire&#34; type, the present invention is equally applicable to other types of brake controls and ABS systems. 
     Briefly, as is well known in the prior art, tractor 12 includes a front steer axle 40, which is usually not driven, and a tandem pair of rear drive axles comprising front-rear drive axle 36 and rear-rear drive axle 38. Wheels 140 and 142 are associated with the rear-rear drive axle 38, wheels 144 and 146 are associated with the front-rear drive axle 36 and wheels 148 and 150 are associated with the front steer axle 40. The rear drive axles, 36 and 38, are driven by engine 46 through transmission 48 and driveline 50. 
     Air actuated brake chambers 60, 62, 152, and 154 are provided for retarding the rotation of wheels 144, 146, 140 and 142, respectively. The rear brakes 60, 62, 152 and 154 are, as is well known in the prior art, of the same size and type as likewise are the front brakes 64 and 66 which are provided for selectively braking the rotation of front steer drive axle wheels 148 and 150, respectively. In the system illustrated, the brakes are air actuated brakes of one of the well known types, such as the &#34;S&#34; cam actuated brake type or the air disc brake type, the details of which are well known in the prior art and may be seen in greater detail by reference to U.S. Pat. Nos. 4,476,968 and 4,457,407, the disclosures of both of which are hereby incorporated by reference. While all of the front brakes and all of the rear brakes should be of the same size and type, it is not necessary, and is not preferable, that the front and rear brakes be of the same size. Compressed air for actuating the brakes is supplied from a plurality of supply tanks 68, only one of which is shown, which supply tanks are provided with compressed air from the vehicle onboard compressor (not shown) or the like. 
     The braking system includes a control unit 70, which for purpose of flexibility and responsiveness is preferably an electronic microprocessor based control unit having means 72 for receiving a plurality of input signals, means for processing input signals in accordance with predetermined logic rules, and means 74 for issuing command output signals to various system operators. 
     A sensor 76 senses the operator&#39;s displacement of a brake pedal 78 to provide an input signal indicative of the driver&#39;s demand or vehicle stopping effort. Sensors of this type are known in the prior art and may be seen by reference to above-mentioned U.S. Pat. Nos. 4,140,352; 4,327,414 and 4,512,615. Typically, such transducers will sense the displacement of and/or force applied to the brake pedal 78 and will provide an output signal proportional thereto. As indicated above, the present invention is equally applicable to more conventional brake systems wherein the amount of braking effort is controlled by a well known treadle valve. To provide tractor ABS operation, wheel speed sensors 160, 162, 98, 100, 102, 104 are provided for providing input signals indicative of the rotational speed of wheels 140, 142, 144, 146, 148 and 150, respectively. 
     The central processing unit 70 will, as is well known in the ABS prior art, process the input signals in accordance with predetermined logic rules to generate command output signals to the front control valve 108 and the rear control valve 168. Briefly, control valve 108 is connected to the supply tank 68 through supply line 110, and, in accordance with the command output signals from CPU 70, independently pressurizes the conduits 116 and 118 leading to air brakes 64 and 66, respectively. Rear control valve 168 is also connected to supply tank 68 through supply line 110 and, in accordance with command output signals from CPU 70, individually provides pressurized fluid via branch conduits to the rear axle brakes 60, 62, 152 and 154. Accordingly, it may be seen that the braking efforts of each of the tractor wheels may be individually controlled for ABS purposes in a closed loop manner in response to command output signals generated by the CPU 70 in response to the input signals received and processed thereby, as is known ABS technology. 
     To provide the trailer anti-swing control system/method of the present invention, the tractor braking system is also provided with one or more sensors 170 and 172, preferably mounted on the tractor such as at the fifth wheel 34, for providing input signals to CPU 70 indicative of and/or allowing the calculation of the current articulation angle (AA), and of time derivatives thereof such as the rate of change of articulation angle (dAA/dt) and the rate of change of the rate of change of articulation angle (d 2  AA/dt 2 ). Of course, sensors may be provided for directly sensing one or more of the time derivatives of articulation angle. In addition, a control valve 174, controlled by command outputs from CPU 70, will provide pilot or controls to the standard trailer brake system control valves. Preferably, the control valve 174 is located on the tractor just upstream of one of the gladhand connectors 206. Valve 174 may be replaced by a standard treadle valve having an output proportional to the displacement of brake pedal 78 and an ABS type valve connected in series downstream therefrom. 
     Applicant&#39;s convention for the measurement of articulation angle, and the first and second derivatives thereof with respect to time, and various sensing means for sensing same will be described in greater detail below. 
     Referring now to FIG. 3, a prior art standard trailer brake system (i.e, non-ABS) is illustrated. Briefly, the trailer includes a king pin 176 for selective engagement and disengagement to the tractor fifth wheel 34 as is well known in the prior art. The trailer includes a supply tank 178 connected to the tractor air system by means of a fluid connection 180. Trailer axles 42 and 44 support trailer wheels 182, 184, 186 and 188, each of which is provided with an air brake actuators or chambers 190, 192, 194 and 196, respectively. Typically, all of the trailer brakes are controlled at the same pressure by means of a relay valve 198 which has an inlet 200 connected to the trailer supply tank 178 and a pilot valve portion 202 for receiving a pilot air signal from the tractor air system by means of connector 204. Each of the trailer brakes is actuated at approximately the same pressure from a single output 208 from a pilot controlled relay valve 198. 
     Connector 204 is designed for connection with connector 206 on the tractor. Briefly, the connectors 204 and 206 and likewise connector 180 and its associated connection with a vehicle air system (not shown) form the fluid connection commonly known as the &#34;gladhand&#34;. 
     Applicant&#39;s convention, which will be used throughout this application, for the measure of the articulation angle (AA) may be best understood by reference to FIG. 4 wherein the pivot axis between the tractor 12 and trailer 14 defined by the fifth wheel 34 and king pin 176 connection is viewed from the top of the vehicle, i.e. with the trailer extending rearwardly from left to right as shown in FIG. 4. The articulation angle between the tractor and trailer, AA, is defined as the included angle between the longitudinal axis 200 of the tractor 12 passing through the king pin/fifth wheel pivot axis and the longitudinal axis 202 of the trailer 14 passing through the fifth wheel/king pin pivot axis. The articulation angle AA is measured from the tractor longitudinal axis 200 to the trailer longitudinal axis 202 and is assigned a positive value in the counterclockwise direction and a negative value in the clockwise direction. In the example shown in FIG. 4, accordingly, the articulation angle AA would have a positive value under applicant&#39;s herein defined convention. 
     Referring to FIG. 6, as vehicle 10 moves generally from left to right as seen in FIG. 6 around the corner defined by the curved lane, it may be seen that the wheels of the trailer 14 lose transverse stability and, under the influence of centrifugal force, and a retarding force provided by the tractor itself the trailer 14 will pivot about the king pin such that articulation angle will typically pass through zero, then change signs as the swing out phenomenon progresses. In a trailer swing event, the trailer 14 swinging out of its lane is a potentially hazardous occurrence. This is an example of the type of trailer swing event that the anti-trailer swing control method and control system of the present invention is intended to prevent, arrest, or to minimize and allow rapid recovery from. 
     The loss of transverse stability of the trailer wheels sufficient to result in a trailer swing event as illustrated in FIG. 6, is typically associated with an application of the trailer brakes resulting in wheel slip exceeding a desirable value and/or total wheel lock. To prevent the occurrence of a trailer brake induced trailer swing event, or to arrest and minimize the extent thereof and to allow quick recovery therefrom, it is desirable that conditions indicative of incipient or actual onset of trailer swing be quickly sensed and that the vehicle brake system react thereto by releasing the trailer brakes allowing the trailer wheels to roll back up towards vehicle speed to minimize the slip and increase the transverse coefficient of friction, and thus increase the centripetal force of the trailer wheels. 
     The trailer anti-swing control method and system of the present invention is operable to sense conditions indicative of incipient, or the onset of, a trailer brake induced trailer swing event by the sensing, memorizing and/or calculating, both current and at a time when the vehicle brakes are applied, at least one or more of the articulated vehicles&#39; articulation angle, and a time derivative of articulation angle such as the first derivative of articulation angle with respect to time and/or the second derivative of articulation angle with respect to time. It is important to sense trailer swing at the initiation or onset, i.e. prior to the trailer swinging towards an almost zero articulation angle position, to have sufficient time to arrest and/or minimize the condition. It is also important to distinguish trailer swing from jackknife as different corrective responses are required. 
     Briefly, one method of sensing an incipient trailer swing event is the sensing of conditions, when the brake pedal is applied, wherein the articulation angle (AA) and a time derivative thereof such as the first derivative of the articulation angle with respect to time (dAA/dt) are of opposite signs (which is an indication that the trailer 14 is swinging outwardly from the center of a curve which the tractor-semitrailer vehicle is traversing) and the absolute value of the second derivative of the articulation angle with respect to time (d 2  AA/dt 2 ) exceeds the absolute value of a predetermined reference indicating that the trailer is swinging rapidly radially outwardly from the center of the curve being traversed by the articulated vehicle. 
     Upon sensing these conditions, the control logic of the present invention will declare the existence of conditions indicative of an incipient trailer swing event and will release the trailer brakes for a given period of time (T) selected to allow the trailer wheels to roll back up towards vehicle speed to increase the transverse stability thereof and preferably minimize or eliminate the trailer swing and allow the trailer wheels to again track on the curve. After the predetermined period of time, trailer brakes will be reapplied at a rate (R) which may be fixed or may vary with measured system variables to a reapplication pressure (P) which is preferably a function of the value of the first and/or second derivative of articulation angle with respect to time at the onset of the trailer swing event. The brakes will be maintained at this pressure peak until such time as the braking event is terminated, i.e., the operator removes his foot from the brake pedal, at which point trailer brakes will again be controlled by the treadle valve until such time as incipient trailer swing is again sensed. The application of pressure to the trailer brakes is controlled by trailer anti-swing control valve 174, an ABS type valve, which varies the pressure applied to the pilot portion 202 of the trailer relay valve 198. A graphic illustration of the pressure applied to the trailer brakes to minimize and recover from a sensed incipient trailer swing event may be seen by reference to FIG. 5. 
     Referring to FIG. 5, at point 204 to point 206 the vehicle brakes are not applied. At point 206, the vehicle operator has fully depressed the treadle valve or brake pedal 78 causing the relay valve 198 in the trailer, under the control of trailer brake control valve 174, to apply the trailer brakes with a high pressure as seen in point 208. If the operator continues to request a relatively high braking effort, trailer brakes will remain applied at a relatively high level from point 208 to point 210. For purposes of this example, at point 210 it will be assumed that the input signals processed by the CPU controller 70 indicate the existence of an incipient trailer swing event. The CPU 70 will then issue command output signals to the trailer brake control valve 174 causing the trailer brakes to be released as may be seen at point 212. The trailer brakes will be maintained in the released position for a period of time T which is selected as a time sufficient to allow the trailer wheels to roll back up to rear vehicle speed. Time T may be predetermined or may vary with the value of sensed inputs to the CPU. Typically, as the trailer wheels are not provided with speed sensors, this is an empirically predetermined value. Applicant has discovered that a period of time from 0.25 to 1.25, preferably from 0.50 to 0.75, seconds is sufficient to allow the trailer wheels to roll back up to approximately vehicle speed and allow the trailer brakes to be reapplied to minimize the stopping distance of the total vehicle 10. 
     Accordingly, at the end of the predetermined period of time T, i.e., at point 214, the trailer brakes are reapplied at a rate R until a pressure P is achieved at point 216, which pressure P is preferably a function of the sensed or calculated first and/or second derivative of the articulation angle at the onset of the trailer swing event. Vehicle brakes will then be maintained at pressure P until termination of the braking event. Applicants have discovered that a rate of reapply R equal to about 10 PSI per second is an effective rate to achieve the pressure P as rapidly as possible without resulting in a repeat trailer wheel transverse loss of stability. 
     Reapply pressure P and/or the rate (R) of reapply pressure are, as stated above, a function of the dynamics of the trailer swing event as sensed by the values of articulation angle, the first derivative and second derivative of articulation angle during the trailer swing event. 
     Briefly, the absolute values of the first and second time derivatives of articulation angle during a trailer swing event are indicative of the loading on the trailer and/or the friction coefficient of the road. The more rapid the change in articulation angle and the rate of change of the change of articulation angle, the more lightly loaded is the trailer and/or the lower the coefficient of friction of the road. Accordingly, for a relatively high first, second and/or higher derivative of articulation angle with respect to time, a relatively lower reapply pressure and rate of reapplying pressure is desirable while with a relatively lower first and/or second derivative of articulation angle with respect to time, a relatively higher reapply pressure may be applied to the trailer brakes without fear of causing trailer wheels to again lose transverse stability. 
     The preferred control method of the present invention symbolically illustrated in a flow chart format by reference to FIGS. 11 and 11A. 
     In the situation of a tractor 12 towing a lightly loaded or unladen trailer 14 and negotiating a left turn curve on a flat relatively low coefficient of friction surface while traveling at a speed below the critical turning speed and then braking; the articulation angle will be initially negative. (The critical turning speed is a function of the square root of the product of coefficient of friction, a gravitational constant and the turning radius). Upon a full application of the vehicle brakes, the vehicle will typically first tend to jackknife, at which time the absolute value a first threshold or reference value (REF 1 ), which is a function of turning radius, for the absolute value of dAA 2  /dt 2  will be exceeded. Within a certain time later (T REF), if a trailer swing is occurring, the value of dAA 2  /dt 2  will reverse in direction and remain reversed and the absolute value of dAA 2  /dt 2  will exceed the absolute value of a second, higher reference value REF 2 , which is also a function of turning radius. 
     Accordingly, upon sensing application of the vehicle brakes, radius of curvature is sensed, calculated and/or estimated by means of a steering angle sensor, by measuring differential speed of the tractor, preferably the steer axle, wheels, or the like. Based upon the estimated radius of curvature, the values of the first and second reference or threshold values, REF 1  and REF 2 , respectively, are calculated. The initial and current value of AA and of d 2  AA/dt 2  is also sensed or calculated. 
     If the initial absolute value of AA is above a minimum value and the initial value of d 2  AA/dt 2  is in the same direction as the initial value of articulation angle and if the absolute value of initial d 2  AA/dt 2  exceeds the absolute value of the first reference REF 1 , a timer is set. Thereafter, if, prior to the timer timing out, d 2  AA/dt 2  changes directions and remains in the changed direction and then the absolute value of current d 2  AA/dt 2  exceeds the absolute value of the second reference, REF 2 , a trailer brake induced trailer swing event is sensed and appropriate corrective action is initiated. If not, the timer is reset and the subroutine exited. 
     The method of determining the radius of curvature, as mentioned in the above paragraph, is considered important: such a determination could be achieved by interrogating the output of a steer angle sensor incorporated in the steering system of the vehicle or by calculating radius of curvature based upon differential wheel speeds as determined by steer axle wheel speed sensors incorporated in the tractor ABS system. 
     An alternate method of sensing the existence of conditions indicative of the onset of a trailer swing event is to sense, during a vehicle braking operation, that (i) the articulation angle (AA) and the rate of change of articulation angle (dAA/dt) are in opposite rotational directions and (ii) that the rate of change of articulation angle (dAA/dt) exceeds the absolute value of a reference value. 
     Upon sensing such conditions, the CPU 70 will command the controller 174 to cause the trailer brakes to be released for a period of time T and then reapplied at a rate (R) to a reapplication pressure (P) one or both of which may be functions of articulation angle (AA) and/or rate of change of rate of change of articulation angle (d 2  AA/dt 2 ) either the latest values or the values at the instant of sensing the conditions. 
     By way of example, by comparing d 2  AA/dt 2  to a yaw reference, which yaw reference is preferably a function of the initial value of AA, it may be determined if the trailer is relatively heavy or light and/or if the road is of a relatively high or low coefficient of friction by determining if it is of a relatively high or low yaw rotational moment of inertia. As is known, for a more heavily loaded trailer and higher friction road surface, the reapply pressure and/or the rate of reapplying the trailer brakes may be higher, to more rapidly stop the vehicle i.e., without inducing transverse instability. 
     A further alternate method of sensing the onset of a trailer swing event is to sense during a braking operation, (i) if the articulation angle (AA) and the rate of change of the rate of change of articulation angle (d 2  AA/dt 2 ) are of opposite signs, (ii) if the absolute value of articulation angle is at least equal to a predetermined value (about 1° to 3°) and (iii) if the absolute value of the rate of change of the rate of change of articulation angle exceeds a reference value. 
     As is known, the use of higher order time derivatives of sensed articulation angle will provide the possibility of earlier sensing of an incipient trailer swing condition, especially for more lightly loaded trailers and/or lower coefficient of friction road surfaces, while the use of lower order time derivatives responds somewhat later in time but will allow a less sensitive, somewhat more reliable, determination of incipient trailer swing especially for more heavily loaded trailers and/or higher coefficient of friction road surfaces. Accordingly, the various logical methods described herein for testing for trailer swing conditions are not considered to be mutually exclusive but may be used in combination with one another. 
     A sensing that articulation angle and a time derivative thereof are of opposite signs, and the time derivative (such as d 2  AA/dt 2 ) exceeds a reference value is indicative that although the vehicle is momentarily properly articulated, the trailer is accelerating in such a manner as to swing-out from behind the tractor. If the absolute value of the time derivative (d 2  AA/dt 2 ) exceeds the reference value, corrective action, as described above, should be taken. 
     Various sensing devices for sensing the articulated vehicle articulation angle, first derivative of articulation angle with respect to time, and/or the second derivative of articulation angle with respect to time may be seen by reference to FIGS. 7-10. Preferably, the various sensing devices include components, all of which, or at least the active ones of which, are mounted on the tractor 12 of the tractor trailer vehicle 10. 
     Referring to FIG. 7, a fifth wheel assembly 34 is modified by the provision of two rollers, preferably spring loaded floating rollers 220 and 222 which are associated with potentiometers or the like and which will be caused to rotate by pivotal movement of the tractor relative to the trailer to provide input signals indicative of articulation angle and/or the first or second derivatives thereof with respect to time. 
     Referring to FIG. 8, an ultrasonic transceiver 230 is mounted to the tractor 12 and will send and receive ultrasonic signals which bounce off predetermined surfaces on the trailer 14 provide an indication of the articulated vehicle articulation angle and/or derivatives thereof. 
     FIG. 9 illustrates a fifth wheel 34 including a magnet carrying cap 232 which will fit over the trailer king pin and which will carry magnetic means for providing a magnetic sensor for sensing articulation angle and/or one of the derivatives thereof. 
     FIG. 10 illustrates a transmitter associated with the gladhand connection at the trailer 14 which will send signals to be received by a direction resolving receiver 240 mounted on the tractor 12. 
     As may be seen, applicant&#39;s have provided an improved trailer brake control system/method for controlling the trailer brake application forces to prevent, arrest or minimize and provide rapid recovery from trailer swing events. 
     While the present invention has been described with a certain degree of particularity, it is understood that the detailed description of the preferred embodiments is by way of example only and that numerous modifications and rearrangements of the components/steps thereof is possible without departing from the spirit and the scope of the present invention as hereinafter claimed.